Flat steel products of the type in question here are typically rolled products such as steel strips or sheets, and blanks and plates produced therefrom.
High-strength flat steel products are of growing significance particularly in the field of automobile construction, since they enable particularly lightweight chassis and bodywork constructions. Depending on their susceptibility to corrosion, their site of use and the aggressiveness of the environment in which they are used, the flat steel products are used with a metallic protective coating. The low weight of high-strength flat steel products contributes not just to the optimal utilization of the technical performance capacity of the particular drive unit, but also promotes resource efficiency, optimization of costs and climate protection.
High strengths can be achieved in flat steel products by addition of relatively large amounts of strength-enhancing alloy constituents and suitable production methods. However, the costs associated with high alloy contents and complex production methods are at odds with the demand for products at minimum cost which can be produced in an economically viable and operationally reliable manner. It is therefore a key aim in the technical field to lower the production costs for high-strength flat steel products by dispensing with costly alloying elements, for example microalloying elements and molybdenum.
Alloying elements which are obtainable at lesser expense and can be added to high-strength steels for compensation of the for a minimization of the costly strength-enhancing alloying elements frequently lead, as a result of a tendency to grain boundary oxidation and to the formation of oxides at the surface of the steel sheet, to deterioration in the processing properties and surface characteristics. Therefore, it is possible with the known inexpensive alloying concepts in many cases to achieve the high strengths required, but not to assure the other properties such as good toughness, good embrittlement resistance characteristics, and optimal suitability for cold forming and for welding, which are expected of the flat steel products envisaged for automobile construction.
Grain boundary oxidation arises at temperatures above 500° C. as a result of penetration of oxygen, through diffusion processes, along the grain boundaries from the surface into the steel substrate. As it does so, the oxygen forms oxides with the elements having a higher oxygen affinity than iron, for example silicon, aluminum, manganese and chromium. This leads to distinct weakening of the grain boundaries and, in the advanced state, to the detachment of whole grains from the structure. Particularly in forming operations, these act as notches and thus lead to premature material failure. The situation is similar for coated material. The notch effect on grain boundary oxidation also exists in the case of coated material. It is additionally the case that the detachment of individual grains or whole grain layers greatly reduces coating adhesion and results in defects in the surface coating that extend as far as complete detachment of the surface coating.
One example of a flat steel product which is intended for automobile applications and is said to have bake-hardening properties is described in EP 2 392 683 B1. The flat steel product known from this publication consists of a steel containing, as well as iron and unavoidable impurities (in % by mass) more than 0.015% and less than 0.100% C, 0.01%-0.3% Si, more than 1.0% and less than 1.90% Mn, 0.015%-0.05% P, up to 0.03% S, 0.01%-0.5% undissolved Al, up to 0.005% N, less than 0.30% Cr, 0.0003%-0.005% B, less than 0.014% Ti, up to 0.1% Mo, up to 0.4% V, up to 0.015% Nb, up to 0.15% W, up to 0.1% Zr, up to 0.5% Cu, up to 0.5% Ni, up to 0.2% Sn, up to 0.2% Sb, up to 0.01% Ca, up to 0.01% Ce and up to 0.01% La. At the same time, the alloy of the steel should fulfill the following conditions: 2.2≤(% Mn+1.3%×% Cr+8% P+150 B*≤3.1 and 0.42≤8×% P+150 B*)≤0.73. B* is calculated by the formula B*=% B+% Ti×0.2025+% Al×0.01 and is set to 0.0022% when this formula for B* gives a value for B* greater than 0.0022%. % Mn, % Cr, % P, % B, % Ti and % Al in the formulae mentioned denotes the respective Mn, Cr, P, B, Ti and Al content of the alloy.
The microstructure of the known flat steel product alloyed in the manner set out above is additionally to have a microstructure composed of ferrite and a second phase consisting of martensite, residual austenite and optionally pearlite or bainite. The area proportion of the second phase is supposed to add up to 3-15 area %, and the proportion of the surface component of martensite and residual austenite in the total area component of the second phase is to be more than 70%. In addition, 50% or more of the area component of the second phase is to be present at a grain boundary triple point, where grains that are in contact with three or more ferrite grain boundaries are defined as second-phase grains present at the grain boundary triple point.
It is apparent from the working examples adduced in EP 2 392 683 B1 that the known flat steel products having such characteristics have tensile strengths of 430-594 MPa and yield strengths of 201-274 MPa. For production of such flat steel products, EP 2 392 683 B1 proposes a process in which a steel slab heated to 1100-1300° C. is hot-rolled at a finish rolling temperature between the Ar3 transition point and the Ar3 transition point +150° C., after the hot rolling is cooled at a cooling rate of at least 20 K/s to 640° C. and then is coiled at 400-620° C. Then the hot strip is cold-rolled with a degree of deformation of 50%-85%. The cold-rolled strip obtained is subjected to an annealing treatment in a continuous galvanizing and galvannealing line at an annealing temperature of more than 740° C. and less than 840° C., then cooled at an average cooling rate of 2-30 K/s from the annealing temperature to the temperature of a galvanization bath which is kept at 450-500° C. The strip which has thus been subjected to annealing treatment runs through the galvanization bath. The flat steel product leaving the galvanization bath is then, in a first variant of the known method, cooled down directly at an average cooling rate of 5-100 K/s to a temperature of 100° C. or, in a second variant of the known process, subjected to an alloying treatment in which it is kept within a temperature range of 470-650° C. for 30 seconds, in order then to be cooled back down to 100° C. at an average cooling rate of 5-100 K/s.